1. Field of the Invention
This present invention concerns a microelectronic device used to emit light radiation and capable of being used, for example, to form the pixels of displays or of screens, and in particular pixels of the OLED type (Organic Light Emission Displays).
2. Description of the Related Art
The screens of the OLED type are flat screens using the OLED property of organic diode luminescence. In order to regulate the luminescence of an OLED diode associated with a screen or display pixel, a current-driven addressing device, incorporated into the pixel, is generally provided.
An example according to previous designs of such an addressing device associated with an electroluminescent diode 10, of the OLED type for example (Organic Light Emission Diode) is illustrated in FIG. 1. This example of an addressing device firstly includes a first transistor 11, operating as a switch, and whose opening or closure is controlled by a selection signal, in the form of a voltage, denoted vlin, for example.
The addressing device also includes a second transistor 12 used to produce a current id at the input of the electroluminescent diode 10, as a function of a control voltage vdat, with the current id provoking the emission of radiation by the diode 10.
The control voltage vdat is a function of a light or luminance intensity value at which it is desired to fix the radiation emitted by the diode 10.
For a certain value of the selection signal vlin, the first transistor 11 can be put into a “ON” state. The control voltage vdat is then applied to the drain of the first transistor 11, and transmitted to the gate of the second transistor 12, with the latter then emitting the current id at the input of the electroluminescent diode 10.
In order to benefit from a maximum of current stability and a minimum of sensitivity to fluctuations of voltage between its drain and its source, the second transistor 12 is generally polarised to saturated state by a polarising voltage for example, denoted Vdd, of the order of +16 V for example.
A capacitor 13, of the order of 1 pF for example, connected to the gate of the second transistor 12, is also provided to allow retention of the control signal vdat, when the latter is transmitted to the gate of the second transistor 12.
A pixel formed from the aforementioned device, has a contrast that is dependent on the extent of the range of light intensities that the diode is capable of producing. In order to allow the diode 10 to attain a large range of light intensities, the second transistor 12 must preferably be capable of sourcing a large range of currents, and be able to produce both “low” currents of the order of a few tens of nanoamperes for example, of the order of 50 nA for example, or “high” currents, of the order of a few microamperes for example, 5 μA in saturation mode for example. The extent of said range of currents, as well as the current values in this range, are dependent in particular on the manner in which the first 11 and the second transistor 12 are polarised.
In an addressing device for a screen or display pixel of the type just described, the first transistor 11 and the second transistor 12 can be transistors of the TFT (Thin Film Transistor) type, manufactured in polycrystalline silicon technology. This type of transistor, frequently used in pixel addressing devices, has some limitations.
Such a TFT transistor is generally limited regarding the extent of the range of current that it is capable of sourcing, in particular in relation to an MOS transistor in monocrystalline silicon technology. This limitation can adversely affect the performance, in particular in terms of contrast, of the pixels using this technology. The TFT transistors in polycrystalline silicon technology also have the drawback of having a slow transition between the cut-off state, which we will call “OFF” and the saturated state, which we will call “ON”.
If we now relate this problem to the case of the addressing device illustrated in FIG. 1, so that the diode 10 can emit radiation with sufficiently high light intensities, then the control voltage vdat must preferably reach high levels too. High values of the control voltage vdat result in high consumption values.
Given the slow transition between the “ON” and “OFF” modes of the TFT polycrystalline silicon transistors, so that the diode 10 can emit radiation according to an extended range of light intensities, the difference between the maximum value, denoted Vdatmax, of the control voltage vdat and the minimum value, Vdatmin, of this same control voltage, is generally large.
So that the diode 10 emits at high light intensities, the voltage between the drain and the source of the first transistor 11 is generally large. This can have as a consequence the occurrence of leakage currents in the first transistor 11. The capacitor 13 used to maintain the control signal vdat at the input of the second transistor 12 can then tend to discharge.
Now poor retention of the control signal vdat at the input of the second transistor 12 can result, for a given pixel, in a random variation in the light intensity emitted by said pixel.
For example, when the second transistor is of the TFT type, polarised with a voltage Vdd of 16 volts, to reach a minimum value of current at the input of the diode 10 of the order of 50 nA, Vdat2min can be of the order of 8, 3 volts for example. To reach a maximum value of current at the input of the diode 10 of the order of 5 μA, the maximum value of the control voltage, denoted Vda2max, can be of the order of 16, 6 volts for example.
The problem arises to improve the performance of the screen or display pixels, of the OLED type for example, in particular in terms of contrast and power consumption. There is also the problem of preventing random variations in the light intensity produced by these pixels.